Chapter 13: Vector-Valued Functions and Motion in Space - Section 13.1 - Curves in Space and Their Tangents - Exercises 13.1 - Page 746: 5

$v(\frac{\pi}{4})=\frac{\sqrt 2}{2}i-\frac{\sqrt 2}{2}j$ $v(\frac{\pi}{2})=j$ $a(\frac{\pi}{4})=-\frac{\sqrt 2}{2}i-\frac{\sqrt 2}{2}j$ $a(\frac{\pi}{2})=-i$

$v(t)=cos(t)i-sin(t)j$ $=>$ $v(\frac{\pi}{4})=\frac{\sqrt 2}{2}i-\frac{\sqrt 2}{2}j$ $v(t)=cos(t)i-sin(t)j$ $=>$ $v(\frac{\pi}{2})=j$ $a(t)=-sin(t)i-cos(t)j$ $=>$ $a(\frac{\pi}{4})=-\frac{\sqrt 2}{2}i-\frac{\sqrt 2}{2}j$ $a(t)=-sin(t)i-cos(t)j$ $=>$ $a(\frac{\pi}{2})=-i$